Remove .rvmrc and add Ruby version to Gemfile.

Expecially because the rvmrc was pointing at an ancient version of Ruby
that I don't even have installed.
Allow events to define a transition_missing hook.

I've added a `transition_missing` hook so that events can implement
their own logic for dealing with events received in an improper state.

For example:

```ruby
class Order
  stateflow do
    initial :received
    state   :received,:paid,:shipped,:cancelled

    event :payment_received do
      transitions from: :received, to: :paid
    end

    event :shipment_sent do
      transitions from: :paid, to: :shipped
    end

    event :cancel do
      transitions from: :received, to: :cancelled
      transitions from: :paid,     to: :cancelled

      transition_missing do |from_state|
        if from_state == 'shipped'
          raise "Can't cancel a shipped order."
        end
      end
    end
  end
end
```

This allows a user to define their own Exception to be raised, or,
in fact any logic (like send another event to move into an error state).
Fix ActiveRecord deprecation warning from old-school scope usage.

Removes annoying deprecation warnings from Rails output:

```
DEPRECATION WARNING: Using #scope without passing a callable object is deprecated. For example `scope :red, where(color: 'red')` should be changed to `scope :red, -> { where(color: 'red') }`. There are numerous gotchas in the former usage and it makes the implementation more complicated and buggy. (If you prefer, you can just define a class method named `self.red`.). (called from add_scope at /Users/jnh/.rvm/gems/ruby-2.0.0-p247/bundler/gems/stateflow-dbc47eed5c77/lib/stateflow/persistence/active_record.rb:12)
```
Merge pull request #39 from jamesotron/no-more-rvmrc

Remove .rvmrc and add Ruby version to Gemfile.
Merge pull request #41 from jamesotron/fix-ar-deprectation-warning

Fix ActiveRecord deprecation warning from old-school scope usage.
Merge pull request #40 from jamesotron/custom-exceptions

Allow events to define a transition_missing hook.
Merge Upstream.